1) Field of the Disclosure
The disclosure relates generally to aircraft air conditioning systems and methods, and more particularly, to systems and methods for improving low inlet pressure cooling performance of an air cycle machine (ACM) pack system in a bleed air based aircraft air conditioning system.
2) Description of Related Art
A typical aircraft air cycle machine package (ACM pack) for an aircraft air conditioning system may include one or more heat exchangers, a compressor and one or more turbines. These components are sized for various cooling and air flow criteria both on the ground and in flight. The power to operate the ACM pack may be obtained from the difference of inlet to outlet pressures with the outlet pressure being defined by the required aircraft cabin pressure. Thus, as the available inlet pressure is reduced, the ACM pack's performance potential is also reduced.
When operating at higher altitudes, the available inlet pressure decreases due to the reduced outside ambient air pressure passing through the relatively fixed pressure ratio of the aircraft's engine bleed air system. As this inlet pressure is reduced to the limits of the pack's cooling and air flow performance, air flow may be reduced to maintain the desired cold output temperature or the output temperature may be raised to maintain the desired air flow by bypassing some flow around the turbine(s). However, as power through the ACM pack is reduced, the compressor and turbine(s) move off their optimal design points and may become less efficient, further aggravating the power loss due to the reduced inlet pressure. This may result in an even greater reduction in the ACM pack's performance with the reduction of inlet pressure.
Recent FAR (Federal Aviation Regulation) requirements regarding minimum air flow per occupant in a passenger cabin of an aircraft, have complicated the issue between cooling performance and air flow when inlet pressures are reduced, in that if air flow is to be reduced, it must be reduced to non-occupant usages such as cargo. Thus, ACM packs need to be designed not only to meet pull down performance but also to meet cooling and air flow requirements at the lower inlet pressures seen at altitude. This may require trading the increased size and weight of a more capable pack versus placing limits on the ACM pack requirements due to heat loads, occupant counts, and or cargo.
Accordingly, what is needed is an improved aircraft system and method for improving low inlet pressure cooling performance of an air cycle machine (ACM) pack system in a bleed air based aircraft air conditioning system, while still maintaining the necessary air flow.